Conventional tentering clamps are equipped with guide rollers for a vertical and horizontal guiding along guide rails in a shrinking machine. Each clamp has a clamp body having connected thereto a clamping section for gripping and holding the edge of a film web. Conventional tentering chains with these clamps also include features for changing the on-center spacing or pitch between neighboring tentering clamp bodies. Articulated connecting devices link a plurality of such tentering clamps to each other to form a tentering chain.
Devices as just described are used for stretching a film web and/or for shrinking a film web in a controlled manner. Several different machines of this type have become known in the art. A basic requirement that must be met by all tentering chains for a controlled stretching and/or shrinking of a film web of thermoplastic material is the fact that the pitch between neighboring tentering clamps must be variable so as to increase during certain time intervals of a full revolution of an endless tentering chain and so as to decrease during other time intervals in the same revolution.
European Patent Publication 0,291,775 (Langer), published on Nov. 23, 1988, discloses an apparatus for the simultaneous biaxial treatment of film webs. European Patent Publication 0,291,775 corresponds to U.S. Pat. No. 4,890,365 (Langer), issued on Jan. 2, 1990. The edges of the film web are clamped in the Langer apparatus by tentering clamps that are mounted to chain links forming chains running around an endless track. The connection between two successive tentering clamps is established by two chain links journalled to each other. These chain links can be brought in pairs into a lengthwise extending arrangement or into a kinked arrangement and vice versa. This positional change of the chain links is controlled by control rails arranged outside the chain guide track, whereby one of the chain links is constructed as a bell crank carrying at a free end of one of its arms a roller engaging the control rail. Thus, the spacing between two successive tentering clamps can be shortened or lengthened, thereby controlling the film stretching and the film shrinking with a fine ratio. The control features in this known Langer apparatus are quite substantial and the linking mechanism is so constructed that the adjustment forces are transmitted horizontally from the control rail through the linking mechanism, whereby the latter must be dimensioned to be capable of not only taking up the adjustment forces, but also the pulling forces that drive the tentering chain.
U.S. Pat. No. 3,195,177 (Kawamura), issued on Jul. 20, 1965, discloses an apparatus for stretching thermoplastic film. The clamping elements for holding the edge or margin of a film web are connected to a linking mechanism capable of adjusting the pitch between neighboring clamping elements. The linking mechanism according to Kawamura is constructed as a toggle lever mechanism that can be effective in a vertical plane or in a horizontal plane. For example, when the adjustment forces are transmitted vertically to the clamping elements, at least one lower and one upper guide is required. The journal points or pivot points of the links forming the linking mechanism are journalled to and guided in the upper and lower guides. The clamping elements also referred to as tentering clamps according to Kawamura are of rather simple construction. However, such simple tentering clamps are not sufficient for meeting the requirements of modern film stretching machines operating at substantial treatment speeds. For example, the following requirements are not satisfactorily met by the Kawamura tentering clamps; a secure clamping of the film edges through the entire treatment operation; a positive, yet easily moving guiding of the tentering clamps; and a small effort and expense for the structural elements that are required for controlling the pitch variations in the Kawamura apparatus. Kawamura also does not show a compact adjustment mechanism for varying the pitch so that a fine tuned variation of the pitch T, particularly a reduction of the pitch T between neighboring tentering clamps is not achievable. Further, Kawamura does not show any features in which the toggle lever of the adjustment mechanism is free of the pulling forces that move the tentering chain at least in those phases of the operation where adjustment forces for a pitch variation do not need to be applied to the pitch adjustment mechanism.